Shachak Pe'eri

LIDAR Technology Applied in Coastal Studies and Management

Abstract The FUDOTERAM is a national Canadian light detection and ranging (LIDAR) project founded by the Canadian Network of Excellence GEOmatics for Informed DEcision (GEOIDE) that investigates data fusion from airborne, marine, and terrestrial mapping sensors. In March 2009, the second Fusion des Données TERrestres, Aériennes et Marines (FUDOTERAM) workshop was held in Quebec City, Quebec, Canada. The focus of the workshop was on international collaboration: Workshops can provide an international platform for sharing ideas and study results among academy, industry, mapping and charting organizations, and service providers. LIDAR work and research included data collected from seven different coastal areas in four nations. This special issue contains selected studies from the second FUDOTERAM workshop on LIDAR technology applied in coastal studies and management. Current studies in this special issue explore LIDAR processing in charting and mapping organizations, shoreline mapping, data integration, coastal processes and coastal management, and seafloor characterization.

The Seafloor: A Key Factor in Lidar Bottom Detection

The environmental factors that determine the ability of airborne lidar bathymetry (ALB) to detect the seafloor are not well understood; however, water clarity is often considered the single factor for detection. A comparison of data from two different ALB systems (LADS-MKII and SHOALS-3000) of a small area offshore Gerrish Island, Maine, USA shows a striking correlation (95% overlap) in areas of no bottom detection that is independent of the tide status, the date of collection and the orientation of the survey flight. The laser measurements from the two ALB systems are compared to acoustic measurements of depth, seafloor slope, and backscatter from a Kongsberg EM3002 echosounder. The comparison shows that in water depths deeper than 7 m, there is a close correlation between the ALB detection patterns and bottom features. The study results indicate that lack of bottom detection by ALB does not necessarily indicate that water depths deeper than the surrounding areas have lidar strong bottom detection. No bottom detection in the study area actually reflects a change in bottom characteristics.

Field calibration and validation of remote-sensing surveys

The Optical Collection Suite (OCS) is a ground-truth sampling system designed to perform in situ measurements that help calibrate and validate optical remote-sensing and swath-sonar surveys for mapping and monitoring coastal ecosystems and ocean planning. The OCS system enables researchers to collect underwater imagery with real-time feedback, measure the spectral response, and quantify the water clarity with simple and relatively inexpensive instruments that can be hand-deployed from a small vessel. This article reviews the design and performance of the system, based on operational and logistical considerations, as well as the data requirements to support a number of coastal science and management projects. The OCS system has been operational since 2009 and has been used in several ground-truth missions that overlapped with airborne lidar bathymetry (ALB), hyperspectral imagery (HSI), and swath-sonar bathymetric surveys in the Gulf of Maine, southwest Alaska, and the US Virgin Islands (USVI). Research projects that have used the system include a comparison of backscatter intensity derived from acoustic (multibeam/interferometric sonars) versus active optical (ALB) sensors, ALB bottom detection, and seafloor characterization using HSI and ALB.

Optical Detector Array Design for Navigational Feedback Between Unmanned Underwater Vehicles (UUVs)

Designs for an optical sensor detector array for use in autonomous control of unmanned underwater vehicles (UUVs), or between UUVs and docking station, are studied in this paper. Here, various optical detector arrays are designed for the purpose of determining and distinguishing relative 5 degrees-of-freedom (DOF) motion between UUVs: 3-DOF translation and 2-DOF rotation (pitch and yaw). In this paper, a numerically based simulator is developed to evaluate varying detector array designs. The simulator includes a single light source as a guiding beacon for a variety of UUV motion types. The output images of the light field intersecting the detector array are calculated based on detector hardware characteristics, the optical properties of water, and expected noise sources. Using the simulator, the performance of planar and curved detector array designs (of varying size arrays) are analytically compared and evaluated. Output images are validated using empirical in situ measurements conducted in underwater facilities at the University of New Hampshire, Durham, NH, USA. Results of this study show that the optical detector array is able to distinguish relative 5-DOF motion with respect to the simulator light source. Furthermore, tests confirm that the proposed detector array design is able to distinguish positional changes of 0.2 m and rotational changes of 10 ° within 4-8 m range in x-axis based on given output images.

Characterization of optical communication in a leader-follower unmanned underwater vehicle formation

As part of the research to development an optical communication design of a leader-follower formation between unmanned underwater vehicles (UUVs), this paper presents light field characterization and design configuration of the hardware required to allow the use of distance detection between UUVs. The study specifically is targeting communication between remotely operated vehicles (ROVs). As an initial step in this study, the light field produced from a light source mounted on the leader UUV was empirically characterized and modeled. Based on the light field measurements, a photo-detector array for the follower UUV was designed. Evaluation of the communication algorithms to monitor the UUV’s motion was conducted through underwater experiments in the Ocean Engineering Laboratory at the University of New Hampshire. The optimal spectral range was determined based on the calculation of the diffuse attenuation coefficients by using two different light sources and a spectrometer. The range between the leader and the follower vehicles for a specific water type was determined. In addition, the array design and the communication algorithms were modified according to the results from the light field.

Distance detection of Unmanned Underwater Vehicles by utilizing optical sensor feedback in a leader-follower formation

This paper proposes an optical detection system between a leader and a follower Unmanned Underwater Vehicle, specifically Remotely Operated Vehicles (ROVs). Cost efficient photodetectors and a single LED light source are used to develop distance detection algorithms to detect translational motion in x-and y-axis directions. Analytical simulations are performed where light is modeled as a first order Gaussian function and integrated into the nonlinear ROV dynamics. The stability of a proportional derivative (PD) controller is shown via Lyapunov stability, as in Fossen [7]. Both leader and follower ROV motions are simulated and experimental results from the distance detection algorithm are shown for proof of concept. In this stage of research, all experiments are performed out of water. Initial results indicate that the proposed detection system shows promise as a precursor stage to underwater testing.

The impact of sea state condition on airborne lidar bathymetry measurements

Due to a large number of available Airborne Lidar Bathymetry (ALB) survey datasets and scheduled future surveys, there is a growing need from coastal mapping communities to estimate the accuracy of ALB as a function of the survey system and environmental conditions. Knowledge of ALB accuracy can also be used to evaluate the quality of products derived from ALB surveying. This paper presents theoretical and experimental results focused on the relationship between sea surface conditions and the accuracy of ALB measurements. The simulated environmental conditions were defined according to the typical conditions under which successful ALB surveys can be conducted. The theoretical part of the research included simulations, where the ray-path geometry of the laser beam was monitored below the water surface. Wave-tank experiments were conducted to support the simulations. A cross section of the laser beam was monitored underwater using a green laser with and without wind-driven waves. The results of the study show that capillary waves and small gravity waves distort the laser footprint. Because sea-state condition is related to wind at a first-order approximation, it is possible to suggest wind speed thresholds for different ALB survey projects that vary in accuracy requirements. If wind or wave information were collected during an ALB survey, then it is possible to evaluate the change in accuracy of ALB survey due to different sea surface conditions.

Pose detection and control of multiple Unmanned Underwater Vehicles (UUVs) using optical feedback

This paper proposes pose detection and control algorithms in order to control the relative pose between two Unmanned Underwater Vehicles (UUVs) using optical feedback. The leader UUV is configured to have a light source at its crest which acts as a guiding beacon for the follower UUV which has a detector array at its bow. Pose detection algorithms are developed based on a classifier, such as the Spectral Angle Mapper (SAM), and chosen image parameters. An archive look-up table is constructed for varying combinations of 5-degree-of-freedom (DOF) motion (i.e., translation along all three coordinate axes as well as pitch and yaw rotations). Leader and follower vehicles are simulated for a case in which the leader is directed to specific waypoints in horizontal plane and the follower is required to maintain a fixed distance from the leader UUV. Proportional-Derivative (PD) control (without loss of generality) is applied to maintain stability of the UUVs to show proof of concept. Preliminary results indicate that the follower UUV is able to maintain its fixed distance relative to the leader UUV to within a reasonable accuracy.

An image processing approach for determining the relative pose of unmanned underwater vehicles

The use of a light source as a beacon is advantageous for the guidance and control of Unmanned Underwater Vehicles (UUVs). This approach allows a follower UUV to determine its relative pose (position and orientation) using low-cost commercial off the shelf (COTS) hardware (e.g., metal halide light sources). In order to design an effective detector unit for the follower UUV and predict its performance, a simulator program has been developed. The program simulates a light field using hardware and environmental parameters describing the light source, water properties, the detector unit geometry and electronic sensitivity. The simulator allows examination of different 3D detector array shapes of varying sizes (physical dimensions and number of detectors). It is convenient to present simulator output as an image, where each pixel represents the intensity logged by a corresponding detector. These image outputs are evaluated for the development of control algorithms for UUVs. Currently control algorithms assume that the water column is uniform with a background noise of known origin. Considered control algorithms are able to provide guidance based on relative intensity values, where the light field samples on the detector array resembles a Gaussian beam pattern. However, disturbances in the medium (e.g., sediment plume) may cause non-uniform distribution of the scatterers that distort the beam pattern. As a result, the control algorithms could misinterpret the acquired image and direct the follower UUV away from the guiding beam. The probability for such a situation increases with distance as the beam diverges. This paper suggests an alternative approach for the development of UUV control algorithms using calculations of various moments of the image (e.g., local Hessian estimations). This method allows the evaluation of the array performance with different array geometries and a varying number of detector elements.

Radiometric and photometric determinations of simulated shallow-water environment

Optical remote sensing is increasingly becoming a preferred economic alternative to the traditional in situ observations and physical sampling for mapping and monitoring habitats. Submersed habitats, such as eelgrass and corals, are especially challenging for field work. Even for remote-sensing work, a priori knowledge of environmental factors is required for highly accurate analysis. Background illumination and water clarity are two key factors that affect the optical remote-sensing imagery, which may vary widely with season, time of year, geographic location, or water depth. This article presents efforts to simulate natural oceanic conditions in a laboratory setting. Solar radiation predicted at different latitudes under varying water clarity conditions and depth were replicated using a 2.5 m deep wave tank at the University of New Hampshire. The goals of the study were: (1) to simulate illumination and water clarity conditions that approximate coastal and oceanic waters, and (2) to quantify the impact of the simulated illumination and water clarity conditions at different depths on the apparent colours that can be observed from an aerial platform. The empirical radiometric measurements included irradiance, radiance, and remote-sensing reflectance from an underwater array of light sources. The results of the study show good correlation (r 2 = 0.89–0.93) between the natural daylight spectrum at the water surface and the irradiance measurements between 350 nm and 590 nm, at 3.5 m from the light array. The colours of the clear and murky water types were photometrically calculated from the radiometric measurements and validated using underwater video imagery. Using this methodology, illumination and water clarity can be replicated under controlled laboratory conditions and used to assist in studying the physical, chemical, and biological processes in habitats, at varied geographic locations and differing environments.